1. Field of the Invention
This invention relates in general to a tunable semiconductor laser and in particular to a laser which can be tuned over a relatively broad frequency range.
2. Description of the Prior Art
For optical communications semiconductor lasers are required wherein the wavelength of the emitted light can be varied within certain limits. This variation in wavelength should occur continuously and very rapidly at electronic speeds. It is desirable to obtain a laser which is tunable that can be manufactured at low cost and which does not change its properties over long operating times.
One possibility of tuning a laser by varying the wavelength is to vary the refractive index in a part of the light conducting semiconductor material by injecting charge carriers. So as to obtain tuning of the laser, a semiconductor laser is divided in two or more sections of different contact regions in the longitudinal directions and these sections are driven with different driving currents. At least one section is designed as a semiconductor laser. The tuning of the wavelength occurs by a variation of the effective refractive index in one or more of those regions which are not designed as semiconductor lasers. The adjustment of the currents through such sections can only occur within narrow limits since the variation of current through a section also influences the operating condition of the other sections. Thus, one disadvantage of the multisection lasers in addition to its complicated manufacturing requirements is the necessity of defining and optimizing the relationship of the various currents to each other on the basis of an extensive measuring program. Also, changes caused by aging to the multi-section laser require that the operating data be reobtained after certain operating times.
Also, for further prior art see U.S. Pat. Nos. 4,352,187, 4,607,370, European Patent Application No. 0 300 790, European Patent Application No. 0 173 269, European Patent Application No. 0 296 066, Article Entitled "Fabrication and Characteristics of MCRW GaAs/GaAlAs Lasers on Semiinsulating Substrate", 8177 Journal of Optical Communications 6(1985) June, No. 2, Berlin, Germany, Japanese Abstract Entitled "Semiconductor Light-Emitting Device" No. 63-272088 (A).